Delete utf8 backfill for old Ruby

Idk if the replacement `require` is necessary exactly, but it's the one-to-one replacement for this lib, so let's start there for now!

app/models/swf_asset.rb

@@ -1,6 +1,6 @@
 require 'fileutils'
 require 'uri'
-require 'utf8'
+require 'encoding/character/utf-8'
 
 class SwfAsset < ActiveRecord::Base
   # We use the `type` column to mean something other than what Rails means!

lib/openneo-auth/session.rb

@@ -1,7 +1,7 @@
 require 'active_support/core_ext/hash'
 require 'msgpack'
 require 'openneo-auth-signatory'
-require 'utf8'
+require 'encoding/character/utf-8'
 
 module Openneo
   module Auth

lib/utf8.rb

@@ -1,13 +0,0 @@
-if String.method_defined?(:force_encoding)
-  # UTF-8 support is native, so let's route the +"abc" syntax to the native
-  # encode method.
-
-  class String
-    def +@
-      dup.force_encoding('utf-8')
-    end
-  end
-else
-  require 'encoding/character/utf-8'
-end
-